1. Field of the Invention
The invention relates to implants within body vessels and more particularly to occlusive devices including embolic coils having stretch resistance.
2. Description of the Related Art
Vascular disorders and defects such as aneurysms and other arterio-venous malformations are especially difficult to treat when located near critical tissues or where ready access to a malformation is not available. Both difficulty factors apply especially to cranial aneurysms. Due to the sensitive brain tissue surrounding cranial blood vessels and the restricted access, it is very challenging and often risky to surgically treat defects of the cranial vasculature.
Alternative treatments include vascular occlusion devices such as embolic coils deployed using catheter delivery systems. In a currently preferred procedure to treat a cranial aneurysm, the distal end of an embolic coil delivery catheter is inserted into non-cranial vasculature of a patient, typically through a femoral artery in the groin, and guided to a predetermined delivery site within the cranium. A number of delivery techniques for vaso-occlusive devices, including use of fluid pressure to release an embolic coil once it is properly positioned, are described by Diaz et al. in U.S. Pat. Nos. 6,063,100 and 6,179,857, for example.
Multiple embolic coils of various lengths, commonly 1 to 30 centimeters, and preselected stiffness often are packed sequentially within a cranial aneurysm to limit blood flow therein and to encourage embolism formation. Typically, physicians first utilize stiffer coils to establish a framework within the aneurysm and then select more flexible coils to fill spaces within the framework. Ideally, each coil conforms both to the aneurysm and to previously implanted coils. Each successive coil is selected individually based on factors including stiffness, length, and preformed shape which the coil will tend to assume after delivery.
During implantation, the physician manipulates each embolic coil until it is in a satisfactory position, as seen by an imaging technique such as fluoroscopic visualization, before detaching the coil from the delivery system. It is highly desired for both ends of each coil to remain positioned within the aneurysm after delivery, because a length of coil protruding into the main lumen of the blood vessel invites undesired clotting external to the aneurysm. After each successive coil is detached, the next coil is at an increasing risk of becoming entangled in the growing mass of coils, thereby restricting the depth of insertion for that coil into the aneurysm.
Difficulties may arise due to stretching of the embolic coils during repositioning or attempted retrieval of the coils, especially if the coil becomes entangled and complete insertion of the coil into the aneurysm is not accomplished. If pulling forces applied to a coil exceed its elastic limit, the coil will not return to its original shape. A stretched coil exhibits diminished pushability or retractability, and becomes more difficult to manipulate into an optimal position or to be removed. Moreover, a stretched coil occupies less volume than an unstretched coil, which increases the number of coils needed to sufficiently pack the aneurysm to encourage formation of a robust embolus positioned wholly within the aneurysm.
There have been a number of attempts to address stretch-related problems in embolic coils. Several stretch-resistant devices are disclosed in U.S. Pat. No. 5,853,418 to Ken et al., having a primary coil and an elongated stretch-resisting member fixedly attached to the primary coil in at least two locations. While Ken et al. mention possible hydraulic delivery of their coils through a lumen of a catheter, they teach that it is desirable to controllably release each coil using a severable or mechanical joint such as an electrolytically detachable joint. Such joints are not compatible with certain delivery systems, and some physicians prefer not to use electrical currents to detach embolic coils from a delivery catheter.
Another embolic device, described in U.S. Pat. No. 6,183,491 by Lulo, has a support wire attached at one end to a proximal end of the coil and attached at its other end to an attachment point located in an intermediate portion of the coil. The embolic device has a closed proximal end and is suitable for hydraulic release from a delivery system after the device is properly positioned. However, only the proximal portion of the coil resists stretching; any length of coil distal to the intermediate attachment point is unprotected from excessive elongation forces.
It is therefore desirable to have an improved stretch-resistant occlusive device which retains flexibility and conformability during insertion into a vascular malformation yet resists stretching along its entire length when pulling forces are applied to it. It is also desirable to have such a device which is compatible with hydraulic deployment systems.